The present invention relates to Moxe2x80x94Sixe2x80x94B alloys and, particularly, Moxe2x80x94Sixe2x80x94B alloys with improved oxidation resistance due to additions of transition elements selected from the group consisting of Fe, Ni, Co, Cu and mixtures thereof.
Molybdenum has excellent high temperature strength which makes it attractive for structural applications at elevated temperatures. The utility of molybdenum and molybdenum-based alloys however are often limited by their poor elevated temperature oxidation resistance. In an oxidizing environment, the first oxidation product that molybdenum forms is molybdenum trioxide. Molybdenum trioxide has a high vapor pressure and sublimes at substantial rates above 1100xc2x0 F., resulting in accelerated metal loss from the alloy. Molybdenum and molybdenum-based alloys are therefore largely limited to use in non-oxidizing environments at elevated temperatures without some form of externally applied oxidation protective coating.
U.S. Pat. Nos. 5,595,616 and 5,693,156 disclose a new class of high temperature oxidation resistant molybdenum alloys, Moxe2x80x94Sixe2x80x94B alloys. In these alloys, the silicon and boron which remain after the initial molybdenum trioxide surface layer volatizes, oxidize to form a protective borosilicate-based oxide scale. If properly processed, these alloys can exhibit mechanical properties similar to other molybdenum-based alloys while also maintaining good oxidation resistance at elevated temperatures (1500xc2x0 F.-2500xc2x0 F.). This combination of mechanical properties and oxidation resistance makes these materials very attractive for high temperature structural applications.
The oxidation resistance of these Moxe2x80x94Sixe2x80x94B alloys is largely a function of the silicon and boron content in the alloy. Increasing the silicon content in the presence of boron, improves the oxidation resistance of the alloy but also results in increased silicide volume fraction. High silicide volume fraction not only makes the alloy difficult to process, it makes it more difficult to achieve mechanical properties equivalent to other molybdenum-based alloys. The ""595 patent discloses that quaternary additions of a variety of elements, specifically C, Hf, Ti, Zr, W, Re, Al, Cr, V, Nb and Ta, could improve the oxidation resistance of the Moxe2x80x94Sixe2x80x94B alloy without increasing the silicide volume fraction. Alloys with the specified quaternary additions exhibited enhanced oxidation resistance at 2200xc2x0 F. and 2500xc2x0 F. relative to the ternary Moxe2x80x94Sixe2x80x94B alloys of equivalent silicide content.
Naturally, it would be highly desirable to further improve the oxidation resistance of Moxe2x80x94Sixe2x80x94B alloys over a wide range of temperature.
Accordingly, it is a principle object of the present invention to provide an improved Moxe2x80x94Sixe2x80x94B alloy that exhibits excellent oxidation resistance at elevated temperatures, that is, temperatures in excess of 2200xc2x0 F.
The foregoing object is achieved by way of the present invention wherein the oxidation resistance of the ternary Moxe2x80x94Sixe2x80x94B alloys are improved at elevated temperatures by minor additions of certain transition elements, such as Fe, Ni, Co, Cu. While earlier alloying additions resulted in the formation of an oxide scale which was protective for tens of hours at 2500xc2x0 F., the described additions result in the formation of an oxide scale which is protective for hundreds of hours (700hrs+) at 2500xc2x0 F. Minor additions of these elements improve the high temperature oxidation resistance of the alloy without any significant effect on the lower and intermediate temperature oxidation resistance of the alloys.